The invention concerns the suspension of motor vehicles. In particular, the invention relates to an axle connecting two wheels, while containing an antiroll function. More specifically, the invention pertains to the category of axles which have two suspension arms, one end of which supports a stub axle receiving a wheel, while the other end is pivotally mounted on the body of the vehicle, and which axle further has a crossmember connecting the two suspension arms.
Several variants of these axles are known and distinguished according to the exact position of the crossmember relative to the arms. This crossmember is sometimes mounted on the body, other times it is mounted in an intermediate position between the axis of the stub axles and the arm pivot axis on the body (see, for example, patent application EP 0,114,790), and still other times it is mounted in the axis of the stub axles, or it is mounted slightly beyond the stub axles, either crossing or not crossing a line intersecting the stub axle axis and the pivot axis, by reason of stresses associated with the space available or by reason of other criteria. This type of axle is commonly found at the rear of passenger cars.
Not considered here are independent wheel suspensions, which sometimes present in their trailing arm variants a sort of perfectly rigid crosspiece, that is, indeformable under the effect of the working stresses. Such a crossmember is placed in the arm pivot axis on the body, and the arms are mounted rotating on the crossmember. Patent application FR 2,393,690 shows an independent wheel axle of this type. The crossmember of such an axle does not enter into the antiroll characteristics of the suspension, as the axle in question cannot be described as flexible.
The invention concerns flexible axles, that is, elastically deformable, in which the deformation or, in general, the stress of the crossmember enters into the antiroll stiffness characteristics of the wheel assembly concerned. In that case, the crossmember, taken as a whole, undergoes a relative rotation of its axial ends on a transverse axis. Patent FR 1,591,438 shows such an axle.
Such a crossmember is dimensioned to be very stiff in flexure. It helps maintain the wheel firmly when the suspension arm in turn undergoes bending stress in a plane containing the wheel arm and the crossmember. Such stresses are due to the transverse gripping of the road by the tire and can become very great on turns negotiated at high speed. In other words, the crossmember helps minimize the difficulty of steering of the wheel, or at least helps to control steering of the wheel, so that it remains within acceptable limits, or so that it can be controlled. In that case, the crosspiece often presents a very high characteristic torsional rigidity.
This is why it has previously been proposed that such crossmember be rendered less stiff in torsion while maintaining its flexural strength at a high level. U.S. Pat. No. 4,787,680 can be consulted in that connection. Unfortunately, such a conception offers sufficient torsion flexibility only when a sufficient length is available to establish the specific zone of the crossmember whose section is suited to reducing the torsional rigidity. In practice, that seems difficult to accomplish except in very heavy vehicles. In fact, the installation of suspension arms having their pivots on the body requires a transverse space practically independent of the size of the vehicle. Therefore, the space available for said specific zone diminishes with the wheel gauge of the vehicle at a rate much greater than proportionally. In other very common applications, the crossmember is formed by an open profile, the torsional rigidity of which is therefore much weaker. One finds, however, that the coupling of such a crossmember to the arms poses numerous problems of endurance. The zone of coupling is the site of a strong stress concentration, which leads to reinforcing it, for example, by welding additional coupling plates.
This is why steering of the wheels (geometric aspect) and control of the body movements (flexibility aspect, clearance of wheels as a function of transfers of loads) are treated separately. An antiroll bar separate from the steering components of the wheel or wheels very commonly gives the axle a roll resistance, added to that coming from the suspension springs through which the load of the vehicle is transmitted to the vehicle.
It is observed in the present state of the art that the choice between independent wheels and flexible axle presents some difficult problems which require compromises in features that are difficult to resolve.
If it is decided to adopt a wheel assembly of the type with flexible axle rather than independent wheels, the design of such axle must satisfy some rather conflicting imperatives. It is necessary to provide sufficient flexural strength for the axle, a guarantee of good maintenance of the wheel surface planes, avoiding excessive steering or steering in an undesired direction due to severe transverse stress. But it is necessary at the same time for the wheel arms to be able to clear each other relatively independently, while preferably having an elastic return to the position where the arms are parallel to each other. That is the antiroll function characteristic of the axle, as mentioned above.